Negative ion source having gas nozzle integral with the cathode



Feb. 2l, 1967 w. D. KILPATRICK 3,305,696 NEGATIVE ION SOURCE HAVING GAS NOZZLE INTEGRAL WITH THE CATHODE Original Filed Jan. 8, 1964 24 /Zg Z 30 wf Mw United States Patent O 3,305,696 NEGATIVE ION SOURCE HAVING GAS NOZZLE INTEGRAL WITH THE CATHODE Wallace D. Kilpatrick, Altadena, Calif., assigner to Electro-Optical Systems Inc., Pasadena, Calif., a corporation of California Continuation of application Ser. No. 336,540, Jan. 8, 1964. This application Oct. 24, 1965, Ser. No. 504,470 11 Claims. (Cl. 315-111) This application is a continuation application of my co-pending application, Serial No. 336,540, led January 8, 1964, now abandoned, by Wallace D. Kilpatrick for Negative lIon Source.

This invention relates to an improved ion source and, more particularly, to a gas discharge type ion source for the production of high output negative ion beams.

Negative ion have several uses. AOne is the production of high velocity neutral atoms which have many end uses in the scientic arts. The production of neutral atoms results when negative ions collide with other elementary particles. Negative ions are used for this pu-r- -pose because the negative ions can make the transition at a lower velocity. The use of negative ions for high velocity neutral atoms is preferably done with hydrogen or with halogens, i.e., fluorine, chlorine, bromine, iodine or astatine. `Negative ions are also used as a beam probe to identify an unknown substance. In this type of application, a negative ion beam passes through some unknown substance in a plasma or gas which modifies the beam. The unknown substance is then identiiied by eX- amining the resultant modification of the beam, the particles in the beam after it passes through the unknown substance may be different velocities for different an-gles of deection. Although neutral or positive ions are also used for this purpose, a negative ion has the advantage that the resultant beam has three different states-the negative, the neutral and a positive state. By subjecting the beam to a magnetic field, the negative state ions detiect to a suitable collector in one position and the positive and neutral ions are deflected to other positions. The ratio of the neutral; negative and positive states of the negative ions after they have passed through the plasma of unknown particles helps further to identify the unknown substance or plasma. Still another use for negative ions is in the implantation of impurities in the semiconductor fabrication art. The impurities are imbedded in the `semiconductor material with the negative ion beam and remain after the beam is withdrawn.

In the past negative hydrogen ions have been produced by streaming electrons through hydrogen gas. These beams were small and were formed in various types of sources, .among which are the oscillating electron sources, `arc discharges, RF discharges and glow discharges. The eliiciency of the formation was extremely low, of the order of 1.0 microampere or less through a .01 square centimeter beam section with a discharge of from to 100 microamps. However, these beams were suitable yfor many laboratory experiments although larger values of negative ion beams could be of use in several elds of applied ion physics. In particular, measurements involving negative hydrogen ion reactions are now possible, in using the present invention, whereas ion beams formed in a previous manner were too weak to permit diagnostics (particle beam probes for plasma or g-as exploration). Other applications are possible in t-he field of thermo-nuclear reactions where methods are used for hydrogen like ion fuels based upon the use of particle charge exchange.

According to the present invention negative ions are extracted from an eliicient oscillating electron arc. In

3,305,696 Patented Feb. 21, 1967 this negative ion source the ions are extracted from the anode region of the discharge Where the polarity of the potentials of the discharge arc is conducive to negative ion removal. Extraction from the anode region is accomplished radially -aeross the magnetic field, and the use of high magnetic elds tends to confine the negatively charged electrons to the arc re-gion where the heavier negative ions are more readily available for extraction. The arc voltage is high, on the order of 1000 volts, so that the average electron energy, on the order of 10 to 20 volts, facilitates the formation of negative ions in the discharge. The ions are extracted from the anode region through a slot with a closely spaced accelerator electrode having a similar slot and having a positive potential relative to the anode. The negative ion beam th-us passes through the slot in a direction normal to the discharge arc region within the anode. Since a large slot can be used in extracting negative ions from the ion region the extraction .area can be greatly increased over the conventional negative ion sources previously available. Not only are larger beams available but also -greater current densities may be realized. For example, in the practice of the present invention negative hydrogen ion currents up to 0.5 milliampere and intensities up to 2.5 milliamperes per square centimeter have been obtained.

It is, therefore, the object of the present invention to provide for an improved negative `ion source.

Another object is the provision of an oscillating electron ion source for negative hydrogen ion production.

Another object is the provision of negative hydrogen ion beams extracted from the anode region radially across the magnetic field used to coniine the negatively charged electrons to the oscillating electron arc region.

Another object is the provision of a gas discharge type ion source for producing high output and intense negative ion beams.

Other objects will become more apparent as a description of the invention proceeds, having reference to the drawings wherein:

FIG. 1 is an elevational view of one embodiment;

FIG. 2 is a plan view taken along the line 2-2 in FIG. 1; and,

FIG. 3 is a schematic illustration of the electrical circuitry used.

Reference is now had to a preferred embodiment shown in FIGS. 1 and 2 wherein there is shown a cylindrical arc anode 10 having exemplary dimensions of an inside diameter of 12.5 millimeters and a length of 67.5 millimeters. Positioned over each of the ends of -anode 10` arc insulators 12, 14 through which are positioned two molybdenum cathodes 16, 18 having an outside diameter of 9.4 millimeters. Although the electron emission is preferably from cold cathode emission Iboth cathodes can be indirectly heated lwith resistance wire elements to temperatures of the order of 300 centigrade. Cathode 18 is shown with such a heater element 20. The cathodes 16, 18 lare of electron emission material such as molybdenum, magnesium or lanthanum boride whereby electron emission may occur under the iniiuence of high ternperature, ion lbombardment of electric eld emission.

Hydrogen or other ionizable gas is introduced into the arc discharge region within anode 10 through a small hole (1 millimeter diameter) in the center of one or both cathodes 1-6, 18 and the entire ion source is Ioriented in a magnetic eld pro-vided by coils 22 so that the direction of axis of symmetry in the magnetic field are co-linear with the axis of the anode 10. A hole is chosen as the means for introducing the gas because, as is obvious, the gas will diverge away from the hole so that neutral particle density is higher at the point of exit from the hole than it is at points away from the hole. Thus, in effect, two pressure areas are created, a higher .pressure area at the hole exit and a lower pressure area at points away from the hole and .a -lower pressure area is created at the point of negative ion extraction as discussed hereinafter. A hole was used in the preferred embodiment because of expediency, however, other means can be utilized to produce the same effect such as a series of distributed holes or a porous crystalline plate and the like.

A slot 24 is provided in the anode 10. The slot preferably is 20 millimeters long by v1 millimeter wide. An accelerator electrode 26, which has a similar slot, is spaced about 2 millimeters from the anode so that the crosssection of the region over which acceleration occurs is approximately 1 by 2 millimeters. A collector 28 which is 100 by 100 millimeters in area and l millimeter thick is preferably spaced approximately 33 mil-limeters from the accelerator and extractor electrode 26. The lbeam collector electrode 28 is at the same electrical potential as the accelerator electrode 26 so that the beam traverses a region which is essentially free from electrical fields.

The resulting oscillating electron cold cathode arc is operated at a high value of about 1000 volts in which the average energy of the oscillating electrons is on the order of 10 to 20 electron volts. Arc currents within anode 10 range from to 100 milliamps ywith the magnetic fields between 300 and 3000 gauss. The arc accelerator 26 and negative ion collector 28 have been assembled in a relatively small rectangu-lar volume of roughly 6 by 8 inches.

'For a higher degree of ionization an auxiliary gas which becomes ionic at a lower voltage may be added. For such purposes an auxiliary gas reservoir, not shown, may be provided wherein a gas such as cesium, for example, may be placed therein. Heat may be used to control the ion population. It has also been found that water vapor added to the hydrogen increases the negative ion yield several times. For this purpose an external opening, not shown, may be provided to communicate with the arc discharge region within anode 10.

Briefiy, the hydrogen arc discharge method employs a very high voltage, efficient, oscillating electron type gas discharge through which a stream of hydrogen gas flows. The arc is operated at a voltage which is efficient for electron attachment and with large electron densities. Negative hydrogen ions formed in the arc are extracted from the discharge in a radial direction normal to the D C. magnetic field Iwhich is coaxial with the arc discharge. The negative ion beam deposits negative charges on the collector plate 28 and the beam 30 is neutralized. The neutralized gas may then be exhausted from the system. If the energy is high enough, hydrogen becomes implanted on the collector plate.

Applicant has constructed a model of the invention in the form of the preferred embodiment shown in the drawing yherein. Test data on this model show the negative ion yield to be higher than can be obtained with prior art devices.

It is believed that the two pressure areas created as discussed hereinabove provide a greater yield of negative ions because of the lack of neutral particles in the low pressure areas; that is, ions are formed in the 'higher pressure area at the gas injection point by electrons in the discharge arc colliding with the neutral particles. These ions have a greater chance of surviving las ions Ibecause they immediately go to a low pressure area, i.e., recollision will not destroy the negative ions already created as will occur if the pressure remained relatively high throughout the ionization chamber. Also positive ions may be formed in the higher pressure region. All of the ions formed are constrained within the low pressure region by the electromagnetic field. Thus, the positive ions formed are maintained at high ionic density in the low pressure area and because of the low pressure, the neutral particle density is lo-w. Some types of positive ions are, it is believed fragmentable; thus, by collision between these positive ions and electrons more negative ions will `be formed than would be the case if the collisions occurred primarily with neutral particles as would be the case if no lower pressure .area were available. In other words, more negative ions will thus be created in the low pressure area because an electron will, so to speak, find a fragmentable ion more readily since the ionic density is high and the neutral particle density is low. Further, more negative ions will ybe produced from collisions with these fragmentable ions than would be produced from the rare three body collisions that would be required were the fragmentable ions not available.

In applicants tests, hydrogen was used as the ionizable gas. Applicant believes that in the immediate vicinity of the gas Ioutlet por-t formed in the cathode, since pressure is high, large numbers of H3+ ions are '.formed. These H3+ ions fragment readily and, therefore, may react with electrons to form negative ions. The H3+ ions are constrained to the proper lower pressure area by the electromagnetic field. The lgas pressure, as can be readily noted, -beoomes rapidly reduced in the anode region away from the cathode because of the divergence of the gas stream away from the gas injection port of the cathode. Thus, as discussed hereinabove, electrons from the arc will collide with such H3+ ions to :form negative ions with a high yield and which ions may 4be efficiently removed from the low pressure area. Further, as discussed hereinabove, the number lof recollisions of the negative ions with neutral particles and other ions will be significantly reduced because Kof the low gas pressure in the extraction area.

Reference is now made to FIG. 3 wherein is shown the relationship of the electrical potential applied to the various elements. For example, the anode 10 is positive relative to the negatively charged cathodes 16, 18 which, together with lthe magnetic field, not shown, constrains the negative ions within the anode `and between the cathodes. Collector 28 and accelerator 26 are at the same potential, as previously explained, and this potential is positive relative to that of the anode 10 to extract the negative ions through slot 24. Battery 31 represents the necessary power supply for this purpose.

Having thus described a preferred form of the present invention it is to be understood that such description was for illustrative purposes only and that the scope of the invention is not to be limited thereby. Other embodiments, alterations and modifications will become readily appa-rent to thiose skilled in the art and such modifications are intended to become part of the present invention as set forth and defined by the following claims.

What is claimed is:

1. An improved negative ion source comprising:

(a) an anode,

(.b) a cathode spaced from said anode, said cathode having a negative potential rel-ative to said anode, whereby an arc discharge is created therebetween.

(c) means for injecting an ionizable gas stream at a first pressure in the vicinity of said cathode and for reducing the pressure to a lower second pressure at an area in the discharge region intermediate said cathode and said anode, whereby said gas is ionized,

(d) means for establishing a magnetic `field axial to said arc discharge to constrain said ionized `gas in said discharge region, and,

(e) means for removing negative ions from said ioized gas at said area of said second pressure in a direction normal to said magnetic field.

2. An improved negative ion source as claimed in claim 1 wherein said anode has an aperture proximate said intermediate area and wherein said removal means comprises an electrode having a potential which is positive relative to said anode.

3. An Iimproved negative ion source as claimed in claim 2 wherein said cathode is located ooaxially within said anode.

4. An improved negative ion source as claimed in claim 3 wherein said anode and said :cathode are cylindrical.

5. An improved nega-tive ion source comprising:

(a) a tubular anode having an aperture therein;

(b) a tubular cathode having a -cover lover one end and its other end adapted for the injection of an ionizable gas thereinto, the covered end; of said cathode being disposed within said tubular anode and spaced away from said ano-de aperture, said cathode having a negative potential relative to said ano-de to crea-te an arc discharge therebetween, said cover having an aperture therethnou'gh for ejection of said ionizable gas linto the arc discharge region, the diameter of said cover aperture being sui'liciently smaller Vthan the inner diameters of said tubular cathode and anode to divergently eject said ionizable gas from said cathode and create a higher gas pressure and neut-ral particle densi-ty adjacent said cover aperture than adjacent said anode aperture;

(c) magnetic means for constraining ionized gas within said anode; and

(d) means for removing negative ions from said gas adjacent said anode aperture.

6. An improved negative ion source as claimed in claim 5 wherein said cathode is located coaxially Within said anode.

7. An improved negative ion source as claimed in claim 6 wherein said anode and said cathode are cylindrical.

8. An improved negative ion source as claimed in claim 7 wherein said ionizable gas is hydrogen.

9. An improved negative ion source comprising:

(a) a tubular anode having an aperture therein;

(b) a plurality of tubular cathodes, each Iof said cathodes having a cover over one end and its other end adapted for the injection of an ionizable gas thereinto, the covered end of each of said cathodes being disposed within said tubular anode and spaced away from said anode aperture, said cathodes having a negative potential relative to said anode to create an are discharge therebetween and resulting oscillating electron trajectories, each of said covers having a rsmall aperture therethrough for ejection of said ionizable gas into the arc -discharge region, the diameter of each of said cover apertures being sufciently smaller than the inner diameters off said tubular cathodes and anode to divergently eject said ionizable gas from said cathodes and create a higher gas pressure and neutral parti-cle density adjacent said cover apertures than adjacent said anode aperture;

(c) magnetic means for constraining ionized gas within said anode; and

(d) means .for removing negative ions from said gas adjacent said anode aperture.

1i). An improved negative ion source as claimed in claim 9 wherein said cathfodes are two in number, said cathodes being mounted coaxially within said tubular anode at opposite ends thereof and with their covered ends facing each other.

11. An improved negative ion source as claimed in claim 5, wherein said means for removing negative ions from said 'gas comprises an accelerating structure disposed in spaced relationship with said anode aperture and maintained at a positive potential relative to said anode to remove nega-tive ions from said electrical discharge, said accelerating structure being so oriented as to extract negative ions from said are region in a direction radial to said magnetic fie-ld direction.

References Cited by the Examiner UNITED STATES PATENTS 2/1950 Backus 250`41.9 l/1963 Flowers 315-111 X OTHER REFERENCES .TAM-ES W. LAWRENCE, Primary Examiner. S. D. SCHLOSSER, Assistant Examiner. 

1. AN IMPROVED NEGATIVE ION SOURCE COMPRISING: (A) AN ANODE, (B) A CATHODE SPACED FROM SAID ANODE, SAID CATHODE HAVING A NEGATIVE POTENTIAL RELATIVE TO SAID ANODE, WHEREBY AN ARC DISCHARGE IS CREATED THEREBETWEEN. (C) MEANS FOR INJECTING AN IONIZABLE GAS STREAM AT A FIRST PRESSURE IN THE VICINITY OF SAID CATHODE AND FOR REDUCING THE PRESSURE TO A LOWER SECOND PRESSURE AT AN AREA IN THE DISCHARGE REGION INTERMEDIATE SAID CATHODE AND SAID ANODE, WHEREBY SAID GAS IS IONIZED, (D) MEANS FOR ESTABLISHING A MAGNETIC FIELD AXIAL TO SAID ARC DISCHARGE TO CONSTRAIN SAID IONIZED GAS IN SAID DISCHARGE REGION, AND, (E) MEANS FOR REMOVING NEGATIVE IONS FROM SAID IOIZED GAS AT SAID AREA OF SAID SECOND PRESSURE IN A DIRECTION NORMAL TO SAID MAGNETIC FIELD. 